1. Sexual Networks in Contemporary Western Societies Fredrik Liljeros Karolinska institutet Stockholm University (Supported by the Swedish Institute for Public Health) S-GEM

2. How is it possible for Sexually transmitted Infections (STIs) to reproduce themselves when the average reported life number of contacts are as low as 6-12 in western societies? Why do we have STIs at all? S-GEM

3. Data sources National surveys + Representative -Validity? -Low Response rate -Only ego network data Clinical data + Network data -Validity? -Representative? Local networks + Network data -Validity? Representative? Online networks + Network data -Validity? Representative S-GEM

4. Representative Sample Population S-GEM

5. Network data vs ego network data Male Female ? ? ? ? ? ? S-GEM

6. Validity Women report twice as few partners on average than men do Find at least five explanations! Low response rate S-GEM

7. Data sources National surveys + Representative -Validity? -Low Response rate -Only ego network data Clinical data + Network data -Validity? -Representative? Local networks + Network data -Validity? Representative? Online networks + Network data -Validity? Representative S-GEM

8. Triangulation S-GEM

9. Basic epidemiology S-GEM

10. Reproduction Rate, R Average number of contacts * probability for transmission * duration of infectiousness Low for STIs ! S-GEM

11. Basic Reproduction Rate, R 0 Gottland S-GEM

12. Reported HIV infected in Sweden www.smittskyddsinstitutet.se S-GEM

13. Reported Chlamydia infected in Sweden www.smittskyddsinstitutet.se S-GEM

14. Explanations for the current situation Probably no single explanation! S-GEM

15. On average, your partners have more partners than you ! Newman Social Networks (2003) May and Anderson Nature (1987) S-GEM

16. The Core-Group Theory

17. Scale free distribution of partners Liljeros et al Nature (2001) National survey data S-GEM

18. Gay men attending a STI clinic in London Colgate et al PNAS (1989) S-GEM

19. Internet contact site data Holme, Edling and Liljeros Social networks 2004

20. Prefferential attachent? S-GEM

21. Concurrency A B C D E AC AB DC DE 2b. Contact network Line graph Morris and Kretzchmar Social Networks (1995 ) S-GEM

22. Concurrency A B C D E Contact network Line graph Kretzchmar and Morris Social Networks (1995 ) ? S-GEM

23. Concurrency A B C D E A B C D E AC BC DC EC 2a. AC AB DC DE 2b. Contact network Line graph Kretzchmar and Morris Social Networks (1995 ) S-GEM

24. Assortative interaction Newman PRL 2002 Liljeros Edling and Amaral (2003) S-GEM

25. Bearman, Moodey and Stovel Forthcoming in AJS (With permission from P. Bearman) Local network motifs S-GEM

26. Australia Thailand USA Holland Norway Spain England Greece Switzerland 231 cases Örebro Austria Falk et al STI (2003) Örebro hospital: Local of infection with Chlamydia S-GEM

27. Conclusions Standard deviation is of high importance Contact tracing are likely to be effective Targeted interventions Further reading…. Liljeros F, Edling CR, Amaral LAN Sexual networks: implications for the transmission of sexually transmitted infections MICROBES INFECT 5 (2): 189-196 FEB 2003 S-GEM

28. Take home message Your partners have more partners than you have yourself! S-GEM